1. Field of the Invention
The invention relates to a pressure frame, designed in particular for an aircraft and consisting of a dome-shaped cover made of composite materials reinforced with fibers, preferably in one single piece, and cover reinforcing devices connected to said cover or produced integrally therewith.
2. Description of the Related Art
For economic reasons, the flight altitude of passenger aircraft is chosen as high as possible, because in higher altitudes there is less air resistance, resulting in less fuel consumption. Common flight altitudes are in the range of 10,000 to 12,000 m (32,808 to 39,370 ft.). Such altitudes, however, are not appropriate for humans, because atmospheric pressure is low, the air does not contain much oxygen, and the air temperature is low. Therefore, the conditions inside the fuselage of an aircraft must be adapted to the living conditions humans are used to. This is done by creating a cabin atmosphere corresponding to an altitude of about 3000 m (9842 ft.). Because the difference in pressure between the inside of the aircraft and its environment corresponds to a difference in altitude of about 7000 to 9000 m (22,966 to 29,527 ft.), the interior of the aircraft must be designed as a pressure chamber which is able to withstand such pressure differences. For this purpose there are so-called pressure frames at the front and rear ends of the fuselage, which are designed to withstand these strains.
The rear pressure frame is usually in the form of a dome-shaped cover, with the concave side facing the aircraft cabin. Known pressure frames consist of a plurality of reinforcing profiles distributed in radial direction and in the direction of concentric circles and are connected to each other e.g. by means of rivets. The skeleton thus formed is covered with overlapping metal parts which are attached to each other and to said reinforcing elements. Therefore, the production of such pressure frames is considerably complicated and costly. If said metal cover starts cracking, there is the risk that the crack will develop further so that the pressure frame will break and pressure will escape from the passenger cabin to the rear, which may lead to a crash of the aircraft. Therefore, in order to make the aircraft as safe as possible, it must be made sure that, if a crack develops, it is prevented from developing further so that pressure can not drop suddenly or like in an explosion. This aim is reached by keeping the sheet metal areas without reinforcing profiles as small as possible; in addition, e.g. crack stoppers made of titanium sheets have to be installed, which makes production even more complicated. In aircraft for several hundred passengers, the rear pressure frame is about 3.5 to 4 m (11.5 to 13,1 ft.) in diameter. Such dimensions result in a relatively high weight of about 100 kg (220 pounds), which has a particularly negative effect because it is far away from the center of gravity of the aircraft.
From EP 387 400 B1 a pressure frame made of composite materials reinforced with fibers for the pressure fuselage of an aircraft is known, which is as light-weight as possible, cheap to produce and easy to install. In order to reach these aims, said pressure frame has a non-uniform fiber layer structure which is more rigid at the periphery than in the center. The periphery of the dome-shaped cover is formed such that it will adapt to the cross-section of the fuselage and is easy to install, e.g. by gluing or riveting. This eliminates the need for an additional frame to install the pressure frame in the aircraft fuselage. The data of the embodiment described show that said pressure frame is intended for relatively small aircraft. In larger aircraft for several hundred passengers some enormous aerodynamic forces act on the sides of the pressure frame, e.g. from the rudder unit forces or thrusts of the power units. In large aircraft the pressure frame is about 3.5 to 4 m (11.5 to 13,1 ft.) in diameter. Such dimensions result in an increased risk of the cover being turned inside out towards the aircraft cabin, i.e. in the direction of the concave side of the cover, if the pressure difference between the cabin and the environment is suddenly reversed. Such reversion of pressure difference may e.g. happen if the aircraft is sinking quickly.
Therefore, the aim of the invention is to provide a pressure frame designed in particular for aircraft; which is as light-weight as possible, may be produced as quickly and cheaply as possible and still meets the strain requirements, particularly for large aircraft. In addition, the risk of destroying the pressure frame by the cover being turned inside out towards the inside of the aircraft is to be reduced. The disadvantages of known pressure frames are to be avoided or at least reduced.
The aim of the present invention is reached by providing a cover of essentially uniform thickness and mounting at least one cover reinforcing device in the central area of the concave side of the cover. Particularly in large aircraft it is necessary and useful to disconnect the cover of the pressure frame from the structure frame, which e.g. has to absorb rudder unit forces or power unit thrusts, by means of a xe2x80x9cresiliently bendingxe2x80x9d peripheral connection so that any side forces can not negatively affect the cover. Therefore, it is useful to attach the cover to the fuselage via a stable profile, which is why said cover need not be more rigid or thicker at the periphery. This, in turn, results in less material being necessary, which means that the structure will have less weight. The term xe2x80x9cessentially the same thicknessxe2x80x9d is meant to include optionally thicker edges around openings or holes in the cover which serve to reinforce these weak points of the cover. The cover reinforcing device provided in the central area of the cover, which device is mounted on the cover on its concave side and is connected to the cover or produced integrally therewith, results in considerably improved stability, because if the pressure difference is reversed, bulging areas will be considerably smaller, thus keeping the cover from being completely turned inside out towards the inside and thus from being destroyed. Appropriate construction of said cover reinforcing device in the central area of the cover allows further reduction in thickness of the cover material without the pressure frame falling short of security requirements.
According to another feature of the invention, said cover reinforcing device located in the central area is formed by a reinforcing structure oriented against the curvature of the cover, for example a truncated cone-shaped, mug-shaped or parabolic structure, with a cavity being arranged between the cover and the reinforcing structure. This provides optimum rigidity, particularly in the central area of the cover, thus reducing the probability of the cover being turned inside out.
Checking the cover of a pressure frame is one of the regular tasks of aircraft inspection. Therefore, according to another feature of the invention, inspection openings for the inspection of the cover part behind the reinforcing structure are arranged in the reinforcing structure in the central area of the cover. These openings will also save material and thus weight. Care must be taken, however, so as that said openings do not substantially reduce the stability of the structure.
Moreover, reinforcing profiles are mounted on the concave side of the cover, particularly in radial direction, which profiles are connected to the cover or produced integrally therewith.
Apart from any connecting elements between the cover and the reinforcing devices or the fuselage, the pressure frame is solely made of plastic material reinforced with fibers. Fiber materials are glass, carbon or aramide (aromatic polyamides). As plastic material, preferably duroplast or thermoplast are used. Fabric is highly tear-resistant and tear-tolerant compared to metal and therefore allows a thinner and thus lighter structure.